1. Field of the Invention
This invention relates, in general, to electro-optical devices and, more specifically, to electro-optical devices utilizing electrochromic materials.
2. Description of the Prior Art
Electrically controllable display devices are widely used as data displays for watches, computers, etc. Typically, these displays include liquid crystal displays, light emitting diode displays, plasma displays, etc.
Liquid crystal displays suffer from the disadvantages of operation over a limited temperature range, substantially no memory and a limited angle of visibility. Light emitting diodes and plasma displays are active devices which require substantial amounts of power and, like the liquid crystal displays, exhibit no memory when power is removed.
Electrochromic devices have been investigated since they afford the significant advantages of memory and reduced power consumption over other types of displays. Such electrochromic devices employ an electrochromic material which is responsive to the application of an electrical field of a given polarity so that the material undergoes a change from a first persistent state in which it is essentially non-absorptive of electromagnetic radiation, that is, it is transparent or bleached, to a second persistent state in which it absorbs electromagnetic radiation and darkens or changes to a colored state, typically dark blue.
In constructing such electrochromic devices, a layer of electrochromic material is disposed, along with an electrolyte or ionic conductive material layer, between two electrodes across which a reversible electric field is applied. When a potential difference of one polarity is applied to the two electrodes, the light absorptive characteristics of the electrochromic layer are altered so that it switches from a clear or bleached state to a darkened state. Reversal of the electric field across the electrodes causes the electrochromic material to revert back to a clear state.
Electrochromic behavior has been noted in a variety of materials, such as the oxides, sulfides and other compounds of transition elements. Extensive use of tungsten oxide (WO.sub.3) has been made in electrochromic devices. However, tungsten oxide while providing long term memory still has drawbacks which has limited its use in certain types of electrochromic devices, such as imaging devices. The response time of tungsten oxide, i.e., the switching time from a clear state to a darkened state and vice-versa, is extremely slow, typically on the order of several seconds. This effectively prohibits the use of tungsten oxide in display applications requiring high switching speeds.
More importantly, repeated switching of the tungsten oxide between clear and darkened states or use over long storage times results in progressive corrosion or dissolution of the tungsten oxide layer which leads to an eventual failure of the device.
In an effort to overcome the problems encountered with the use of tungsten oxide, iridium oxide (IrO.sub.2) has been investigated for use;in electrochromic based devices. Iridium oxide exhibits long-term memory and has a significantly faster switching time, on the order of 50 milliseconds. Further, iridium oxide is resistent to corrosion or dissolution over long-term use.
Despite the advantages associated with the use of iridium oxide as an electrochromic material, the development of a viable electrochromic imaging device utilizing iridium oxide has not, until recently, been possible due to difficulties in applying iridium oxide to electrodes. Further, previous efforts have been directed solely at constructing display devices, such as watches, calculators, etc.; not imaging-type devices.
Thus, it would be desirable to provide an electrochromic imaging apparatus using iridium oxide as the electrochromic material which overcomes the problems encountered with previous electrochromic imaging devices. It would also be desirable to provide an electrochromic imaging apparatus using iridium oxide which is usable as a building block or modular component for many different imaging applications. Finally, it would be desirable to provide a electrochromic imaging apparatus using iridium oxide which exhibits fast switching characteristics, long-term memory and extensive useful life.